Pressure change intrusion detector

ABSTRACT

Apparatus utilizing change in pressure condition in an area to be protected to indicate intrusion or change in conditions of the area by using a pressure detection device to detect pressure waves at low and infrasonic frequencies which includes a pressure transducer sensitive to change in pressure to provide an output signal. In some instances an amplifier is provided to amplify the output signal, a signal conditioning circuit is provided to delay a portion of the signal, and a detector device is provided to detect the signal with a comparator provided to compare the signal with the delayed signal and actuate an alarm. 
     .Iadd.Additionally, a device producing signals responsive to pressure waves and selected sonic frequencies is taught. If the signals are coincidently received at a coincidence detector, an alarm is activated. Further, the signals can be stretched and/or delayed before receipt at the coincidence detector. .Iaddend.

This application is a .Iadd.reissue application for U.S. Pat. No.4,928,085, issued on May 22, 1990, from application Ser. No. 359,045,filed May 30, 1989, which was a .Iaddend.continuation of applicationSer. No. 469,089, filed February 23, 1983, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to method and apparatus which isparticularly useful in connection with the detection of intrusion into aspace to be protected and is particularly useful in detecting, forexample, the opening of a window, the breaking of a window or even asudden impact on the walls of the area.

Heretofore, intrusion detection has been accomplished primarily by meansof audio detectors which detect the occurrence of a sound. In moresophisticated form the detectors can be tuned to detect sounds atspecific frequencies such as the frequency of the sound made by thebreaking of a pane of glass, the slamming of a door or otheroccurrences. The difficulty with audio discriminator detectors is thatin general, there are other occurrences which generate sounds in thesame frequency as the frequency generated by the occurrence to bedetected thus leading to false trips and false alarms.

Infrared detectors are sometimes utilized as intrusion detectors but donot detect the occurrence surrounding the intrusion, such as thebreaking of glass or the opening of a door nor do they detect the eventswhich can occasion the generation of a low frequency pressure wave.

No prior art device is known utilizing a change in pressure as indicatedby the occurrence of a low frequency pressure wave in the area to beprotected as the basis for the detection of the selected occurrenceinvolving change in the physical characteristics of the area to beprotected. Furthermore no prior art devices are known wherein a pressuretransducer is utilized to detect the occurrence of a low frequencypressure wave in an area to be detected accompanied by a specific soundoccurrence such as the breaking of glass, etc.

SUMMARY OF THE INVENTION

The present invention relates to intrusion detection devices and moreparticularly to detection devices actuated by the occurrence of a lowfrequency pressure wave which has been unexpectedly found to accompanyeven the slightest intrusion and has further been found to be extremelyaccurate in detecting the occurrences.

More particularly, the present invention provides an apparatus utilizingchange in pressure condition in an area to be protected to indicateintrusion or change in conditions of the area by means of a pressuredetection device to detect pressure waves at low and infrasonicfrequencies which includes a pressure transducer sensitive to change inpressure to provide an output signal. In some instances an amplifier isprovided to amplify the output signal, a signal conditioning circuit isprovided to delay a portion of the signal, and a detector device isprovided to detect the signal with a comparator provided to compare thesignal with the delayed signal and actuate an alarm.

Examples in accordance with the present invention are shown in theaccompanying illustrations and described herein but it will beunderstood that the examples presented are by way of illustration onlyand are not in any way intended to limit the scope of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples in accordance with the present invention are illustrated in theaccompanying figures wherein:

FIG. 1 is an illustration of a very simple arrangement within the scopeof the present invention which has been found to operate satisfactorilyto detect a selected occurrence;

FIG. 2 is a flow diagram of a somewhat more complicated arrangementwithin the scope of the present invention;

FIG. 3 is detailed schematic diagram of an example of one circuit withinthe scope of the present invention useful in the system of the typeshown in FIG. 2;

FIG. 4 is a flow diagram of a combined pressure sensitive and sonicsensitive device within the scope of the present invention;

FIG. 5 is a detailed schematic of one circuit within the scope of thepresent invention useful in the system of the type shown in FIG. 4;

FIG. 6 is a flow diagram of a dual pressure sensing device within thescope of the present invention; and

FIG. 7 is a detailed schematic diagram of an example of one circuitwithin the scope of the present invention useful in a system of the typeshown in FIG. 6.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a simplified device illustrating oneprinciple in accordance with the present invention. FIG. 1 is anillustration of a device to provide full wave detection of low frequencypressure waves. As previously stated it has been unexpectedly found thatin the event of even the most minor occurrence in a space to beprotected, a low frequency pressure wave is generated which can bedetected indicating the occurrence.

In FIG. 1 a casing 1 is illustrated having a diaphragm 2 in the centerof which is an electrical contact 3. It will be recognized thatdiaphragm 2 moves in response to the direct incidence of a pressurewave, the degree of movement depending upon the characteristics ofdiaphragm 2. Diaphragm 2 can be designed to respond to an extremely lowpressure differential between the area outside the casing and thechamber defined by the casing. A barometric pressure equalizing aperture11 is provided to equalize barometric pressure changes but small enoughto permit measurement of relatively sudden pressure changes. A pair ofcontacts 4 and 6 are provided and carried by a post 10 within thechamber defined by casing 1 where connecting leads 7 and 8 are providedto extend through an insulator 9 in the rear wall of casing 1. In thisform the detector device would provide half wave detection of a pressurewave. Also in accordance with the present invention, full wave detectioncan be provided by means of contacts 12 and 13 carried by post 14outside of casing 1 where contact 3 moves between both contacts 4 and 6and contacts 12 and 13 upon the incidence of a pressure wave to detectfull wave occurrences. Connector 16 and 17 are connected to leads 7 and8 and connected in series through a battery 18 and an alarm 19 forexample, a bell.

It will be understood that where only positive or negative pressure isto be detected the appropriate set of contacts could be disconnected.

FIG. 2 is a flow diagram of a simplified all electronic pressuredetector. A transducer 21 is provided which may include any of a numberof devices suited for this purpose such as, for example, a carbon pile,Piezo-electric, a semi-conductor strain gauge, a capacitance device, areluctance device.

In the device shown in FIG. 2, a low frequency pressure wave 20 isreceived by transducer 21. A signal is fed to an amplifier 22 wheresensitivity of the device is adjusted by means of a potentiometer P1. Anoutput signal is supplied to a signal conditioner 23, as describedhereinafter, to condition the signal to a useful form and theconditioned signal is provided to a rectifier 24 which then provides arectified signal to tracking detector 25 which then provides an outputsignal to a driver 26 which provides an output signal 27 utilized toprovide an alarm.

One device in accordance with the arrangement of FIG. 2 is illustratedin FIG. 3 where a pressure transducer 31 is provided. A source of powerA is supplied through resistor R1 to microphone 31 which is grounded atG. A coupling capacitor C1 is provided and the signal from capacitor C1is provided to potentiometer P1 (as shown in FIG. 2) through a resistorR2 and R3 to the noninverting input of an operational amplifier 32 wherea low pass filter R3-C3 to ground G is provided. A gain circuitincluding resistors R4 and R6 is provided to the inverting input ofamplifier 32 along with a capacitor circuit including capacitor C2. Theamplifier 32 and the associated circuitry including C1-P1, provides aband pass amplifier to pass selected signals, for example between 0.002and 10 cycles per second. The output from amplifier 32 is coupled to theinverting input of a second amplifier 33 by means of a series couplingcircuit including resistor R7 and capacitor C3-C4. A reference signal Bis provided at the noninverting input of amplifier 33 and a capacitorC10 and resistor R8 are provided around amplifier 33 to provide a secondlow pass filter. The output from amplifier 33 is coupled throughresistor R9 to the inverting input of a phase inverter 34 having a unitygain by means of resistors R9 and R11 to provide an output signal 180°out of phase with the output from amplifier 33. Full wave detection isaccomplished by rectifying the in phase signal from amplifier 33 bydiode D1 and the inverted other half of the signal (other phase) fromamplifier 34 by diode D2. The combined outputs from amplifiers 33 and 34are connected through Diodes D1-D2 to ground through resistor R5.Resistor R13 is provided to assure comparator amplifier 36 which isconnected as a comparator is quiescent in the proper state. Resistor R12and capacitor C6 are provided to delay the signal from diodes D1, D2 tothe inverting input of comparator 36 so the device acts as a trackingdetector. That is, the direct signal from amplifiers 33, 34 is providedthrough diodes D1, D2 to the noninverting input of comparator 36 and thecomparator reference signal is delayed by means of the R12-C6 circuit sothat slowly changing (noise, thermal or otherwise) signals do notactuate the device. Accordingly as the input to the noninverting inputof comparator 36 rises slowly, the inverting input tracks thenoninverting input except when a fast changing signal is received totrigger the comparator and turn on the output of comparator 36. Thuswhen comparator 36 is actuated, the output goes high and is supplied toan inverter 37 so that the output of inverter 37 goes low causingcapacitor C5 to be discharged through diode D3. Discharging capacitor C5causes the output 27 of inverter 38 to go high for a fixed minimumperiod of time (stretch) determined by the R14, C5 time constant toactuate the alarm device to indicate the presence of a low pressurewave, capacitor C5 being recharged through resistor R14 to supplyvoltage A.

FIG. 4 is a flow chart of a combined pressure and sound activated devicewhere the same pressure wave sensing arrangement as shown in FIG. 3 canbe utilized including a pressure transducer connected through asensitivity adjust potentiometer P2 to an amplifier 44 which supplies asignal to a signal conditioner 46 where the output of signal conditioner46 is supplied to a full wave rectifier 47 and thence to a trackingcomparator 48 and a holdoff 45 to an AND gate 56. In the sound detectingside of the arrangement a signal 42 is received by a transducer, forexample a microphone 40, which supplies a signal to an amplifier 51through sensitivity potentiometer P3. The output from amplifier 51 issupplied to a signal conditioner 52 which supplies a conditioned signalto rectifier 53 which supplies a rectified signal to a hold off 54 whichlikewise supplies a second signal to AND gate 56. Upon coincidence ofthe signals from holdoffs 45 and 54 AND gate 56 is actuated to activatean alarm 57. A schematic illustration of the arrangement shown in FIG. 4is shown in FIG. 5.

Briefly, in addition to the pressure detector described in FIG. 3 asound discriminator is used to reduce the incidence of false alarms andglass breakage detectors.

In operation the glass breakage detector responds to both the sound ofthe glass breaking and to the low frequency pressure wave generated whenthe window explodes into the room being protected. Holdoff and stretchcircuits described hereinafter are used to insure the sound signal andthe signal generated by the pressure wave are concurrent in the event ofbreakage of the window being protected. It has been found that thepressure wave travels slightly slower than the sound waves so provisionmust be made to both delay and stretch the signals so that in anyreasonably sized area and at any reasonable distance, the signals willbe coincident to trip the device. Accordingly, if a false trip wouldoccur in a sound actuated device, for example, by a dog barking, ortelephone bell, or lightning or other similar occurrence without theoccurrence of the pressure wave, the device will not actuate. On theother hand, when glass breaks or other selected events occur; thesimultaneous pressure wave and sound frequency will actuate the device.In FIG. 5 a pressure transducer 61 is provided with a sensitivityadjustment potentiometer P4. The output 62 from transducer 61 is coupledto a second potentiometer P5 which adjusts the level of the pressuresignal to be received by the pressure detecting device. The pressuresensing circuit in FIG. 5 is a somewhat simplified version of that shownin FIG. 3 where the device shown in FIG. 5 bears similar numerals.

The signal to the sound sensing device is provided through potentiometerP4 where a sound discriminator device described hereinafter is shown toact as a band pass filter and detector to provide actuation in receiptof selected frequency signals. An example of one suitable type offrequency responsive detector is shown in FIG. 5 but it will beunderstood that within the scope of the present invention other similardevices may be found.

Referring to FIG. 5 for an illustration of a sound discriminator devicewhich can be utilized within the scope of the present invention,transducer 61 is shown with the output 62 supplied through potentiometerP4 and a coupling capacitor C9 through resistor R17 to the noninvertinginput of a preamplifier 63. A reference voltage is provided from sourceB through resistor R19 and a gain circuit including resistor R18 isprovided around amplifier 63. The output from amplifier 63 is coupledthrough a high pass filter C10-R19 to the noninverting input of anamplifier 65 supplied with a reference input B through resistor R21. Again circuit is provided around amplifier 65 utilizing resistor R20. Ahigh pass filter including capacitor C11 and resistor R22 is provided onthe output from amplifier 65. A second high pass filter capacitor C12resistors R23-R24 is provided to provide a signal to a rectifier circuitincluding diodes D6 and D7. A signal then results from the charge ofcapacitor C13 through resistor R24.

Because of the speed generally encountered in the occurrence of a soundsuch as breaking glass and the unexpectedly low rate of transmission ofthe low frequency pressure wave, it is necessary to delay the signalgenerated by the sound portion of the .[.occurance.]. .Iadd.occurrence.Iaddend.so a signal extension circuit including diode D12, resistorR37, capacitor C16 to ground G is provided to stretch the signal, forexample for approximately 40 milliseconds. The signal from inverter 68is supplied through a resistor R38 which forms part of an AND gate asdescribed hereinafter.

The signal provided by tracking detector 36 is supplied to a holdoffcircuit consisting of a diode D13 and a resistor R39 and inverter 71where the output from inverter 71 passes through a second "stretch out"circuit diode D14 Resistor R41 and capacitor C17 to ground G to a secondinverter 72. The output from inverter 72 is supplied by means of a diodeD16 which, with resistor R38, provides an AND gate. So long as there isno alarm from sound or pressure the output from inverters 68, 72 is low.An AND gate composed of resistor R38, diode D16 operates as follows:Assuming inverter 72 is low diode D16 will shunt any output frominverter 68 to ground through resistor R38. .[.Conversly.]..Iadd.Conversely .Iaddend.if the output of inverter 72 is high, diodeD16 is reversed biased if the output of inverter 68 is low and in thesetwo instances the anode of diode D17 remains low and there is no output.Upon the occurrence of a pressure wave the output from inverter 72 goeshigh for a short period determined by the "stretch out" circuitD14-R41-C17. In the event of a sound occurrence output of inverter 68goes high. The coincidence of the high signals causes diode D17 toconduct charging C17 through resistor R38. Diode D17 resistor R42 andcapacitor C18 comprise another stretch circuit which determines theminimum duration of the alarm output. The signal is supplied to aninverter 73 where the output from inverter 73 is supplied through aresistor R43 to the base of a transistor Q4 where the emitter isgrounded and the collector is connected to supply voltage A by means ofresistor R44. Transistor Q4 is normally conductive but upon theoccurrence of a simultaneous pressure and sound signal inverter 73 goeslow so that transistor Q4 goes nonconductive and the collector oftransistor Q4 is supplied to the base of transistor Q5 which goesconductive activating a LED 4 to indicate an alarm.

FIG. 6 illustrates another utilization of pressure detectors within thescope of the present invention and in this configuration two separatedetectors are connected in an anti-coincidence mode. Devices of the sortdescribed herein are useful where pressure events normally occur in thespace to be monitored, for example the activation of heating,ventilating, air conditioning equipment, thunderclaps or otheroccurrences where it is desired to compare the pressure waves in thearea to be protected with a reference area so that simultaneousgeneration of in phase pressure waves prevents false tripping of thealarm system. In the arrangement shown, a remote transducer 76 islocated, for example on one side of the wall outside the area to beprotected. A master detector 86 is located in the area to be protected.The phase of the pressure signals are split and the phases are "anded"so that in order to get an output from the master device the remotedevice must generate a signal that is out of phase with the output ofthe detector in the area to be protected. It has been found that thisprocedure minimizes the occurrences false alarms due to common modechanges.

A flow chart of the arrangement is shown in FIG. 6 where the remotelocated transducer 76 is shown connected to an amplifier 77 with thesensitivity adjusted by a potentiometer P11. The output from amplifier77 is supplied to a signal conditioning device 78 which supplies asignal to a phase splitter 79 which provides outputs 81-82 to positiveand negative rectifiers 83 and 84 each of which provides a signal 93-94.In the master detector which is located in the area to be observed,transducer 86, is located to supply a signal to an amplifier 87 withsensitivity adjusted by potentiometer P12. The output from amplifier 87is supplied through conditioning circuit 88 to a phase splitter 89 whichsupplies signals 91-92 to rectifiers 96-97 which supply signals 98-99respectively to the AND gates 101 and 102 along with signals 93 and 94from phase splitters 83 and 84. The outputs of the AND gates 101 and 102are supplied to an OR gate 103 to activate a driver 104 to activate thealarm device.

A schematic illustration of a circuit to accomplish the objective shownin FIG. 6 is illustrated in FIG. 7 where transducers 76,86 are shownconnected to potentiometers P11 and 12. The output from potentiometerP11 is connected through Pdecoupling capacitors C21-C22 and throughresistor R46 to a band pass amplifier 106 which is provided with gaincircuit including resistors R48 and R53. A low pass filter resistor R47,capacitor C24 connected to G is provided to the noninverting input ofthe amplifier. The high pass function of amplifier 106 is provided bycapacitor C23. The inverting input to amplifier 106 is supplied from areference source including a unity gain operational amplifier 107 havinga noninverting input from supply voltage A through a voltage dividerincluding resistors R49-R51 and filtered by capacitor C26. The invertinginput to op amp 107 is provided from its output. The output 108 fromamplifier 107 is supplied through a resistor R53 to the inverting inputof operational amplifier 106 and to the noninverting input of amplifier106 through R52, R46 and R47. The output from amplifier 106 is suppliedthrough a resistor R54 and decoupling capacitors C27-C28 to theinverting input of an operational amplifier 109 to provide an output 110which is of a first phase. A gain circuit including resistor R56, and alow pass determining capacitor C29 is provided for amplifier 109. Withinthe scope of the present invention an inverted phase is provided bymeans of a second operational amplifier 111 where the inverting input toamplifier 111 is provided from amplifier 109 and the noninverting inputto amplifier 111 is provided by output 108 of the reference generatoramplifier 107 and filtered by capacitor C31 and resistor R66. ResistorsR57-R58 are provided in series between the outputs 110 and 113 and arechosen to cause amplifier 111 to have unity gain. Output 113 is providedthrough resistor R59 to the base of a transistor Q6. The output 110 islikewise provided through a resistor R61 to the base of a transistor Q7.Each of the transistors Q6-Q7 transmits the occurrence of a pressuresignal of different phase. The collector of transistor Q7 is connectedthrough an inverter 113 to a holdoff circuit including a diode D16,resistor R63, capacitor C32 to an inverter 116 which supplies an outputsignal to a "stretch out" circuit including diode D18, resistor R67 andcapacitor C34 to provide signal 93 as shown in FIG. 7.

Likewise, transistor Q6 has its collector connected to an inverter 114where the output is connected to a holdoff circuit including diode D17,resistor R64, and capacitor C33 and to inverter 117 which supplies a"stretch out" circuit comprising diode D19, resistor R66 and capacitorC36 so that signal 94 is provided as shown in FIG. 7.

The transducer 86 located in the area to be monitored is connectedthrough potentiometer P12 and decoupling capacitors 37-38 and resistorR71-R72 to a band pass amplifier 121 which is provided with gain circuitincluding resistors R77, R78. Capacitor C41 is used to form a high-passfilter. A low pass filter resistor R72, capacitor C37 connected to G isprovided to the noninverting input of amplifier 121. The inverting inputto amplifier 121 is supplied from a reference source including a unitygain operational amplifier 122 having a noninverting input from supplyvoltage A through a voltage diver including resistors R73-R74 andfiltered by capacitor C42. The inverting input to op amp 122 is providedfrom its output 123. Output 123 from amplifier 122 is supplied through aresistor R77 to the inverting input of operational amplifier 121 and tothe noninverting input of amplifier 121 through resistors R76, R71 andR72. The output from amplifier 121 is supplied through a resistor R79and decoupling capacitors C43-C44 to the inverting input of anoperational amplifier 126 to provide an output 127 which is of a firstphase. A gain circuit including resistor R81, and a low pass determiningcapacitor C46 is provided for amplifier 126. Within the scope of thepresent invention an inverted phase is provided by means of a secondoperational amplifier 131 where the inverting input to amplifier 131 isprovided from amplifier 126 and the noninverting input to amplifier 131is provided by output 123 of the reference generator amplifier 122 andfiltered by capacitor C50 and resistors R80. Resistors R87-R88 areprovided in series between the outputs 127 and 132 and are chosen tocause amplifier 131 to have unity gain. Output 132 is provided throughresistor R91 to the base of a transistor Q9. The output 127 is likewiseprovided through a resistor R82 to the base of a transistor Q8. Each ofthe transistors Q8-Q9 transmits the occurrence of a pressure signal ofdifferent phase. The collector of transistor Q8 is connected through aninverter 128 to a holdoff circuit including a diode D29, resistor R84,capacitor C47 to an inverter 129 which supplies an output signal to a"stretch out" circuit including diode D21, resistor R86 and capacitorC48 to provide signal 98 as shown in FIG. 7.

Likewise transistor Q9 has its collector connected to an inverter 133where the output is connected to a holdoff circuit including diode D62,resistor R93, and capacitor C19 and to inverter 134 which supplies a"stretch out" circuit comprising diode D23, resistor R94 and capacitorC51 so that signal 99 is provided as shown in FIG. 7.

The signals 93-94 and 98-99 are then provided through inverters 171-174to transistors Q14, Q13, Q12 and Q11. The outputs from transistors Q12and Q14 are combined through diodes D28 and D29 to provide an output121. One of the AND gates, for example AND gate 102, of FIG. 6 isprovided by diodes D28,D29. For example, when both transistors Q14 andQ12 go conductive, diodes D29 and D28 are reverse biased, allowingcapacitor C53 to be discharged through resistor R97 causing the outputof inverter 142 to go high. Output 121 is supplied through a circuitincluding capacitors C53, resistor R97 to an inverter 142 and a diodeD32. Output 122 is provided through a delay circuit including resistorR96 and capacitor C52 to an inverter 141 and a diode D31. Thecombination of Diodes D31-D32 provides the OR gates of FIG. 7. Thus uponthe occurrence of a signal in either output 121 or output 122 inverter144 goes low through a "stretch out" circuit including diode D32,resistor R98 and capacitor C54 to an inverter 146 and a resistor R99 toactivate a transistor Q16 which goes conductive and activates an alarm160 and light emitting diode LED 6. A double pole switch 161 is providedto operate from position shown in FIG. 7 to permit operation of theaudible alarm to a second position shown by dotted line where only theLED 6 is activated.

The invention claimed is:
 1. A signalling device to detect coincidenceof first and second selected conditions in a selected area .[.where.]..Iadd.wherein .Iaddend.the first selected condition is infrasonic airpressure waves having frequencies below 20 cycles per second and thesecond selected condition is selected sonic frequency waves, said devicecomprising; a pressure sensitive transducer means responsive to saidinfrasonic air pressure waves to provide an alternating electricalsignal; an amplifier means to selectively amplify said alternatingelectrical signal to provide an amplified alternating output signal; arectifier means to rectify said amplified alternating output signal toprovide a first .Iadd.rectified .Iaddend.signal; a sonic frequencytransducer means responsive to selected frequencies to provide a secondalternating electrical signal; a second amplifier means to selectivelyamplify said second alternating electrical signal to provide a secondamplified alternating output signal; a second rectifier means to rectifysaid second amplified alternating output signal to provide a secondrectified signal; a switch means, including a coincidence detectorresponsive to the first and second rectified signals, which activates analarm in response to coincident receipt of the first and secondrectified signals.
 2. A signalling device according to claim 1 including.[.first.]. delay means to delay transmission of said first.Iadd.rectified .Iaddend.signal to said switch means for a selectedperiod of time.
 3. A signalling device according to claim 1 including.[.second.]. delay means to delay transmission of said second.Iadd.rectified .Iaddend.signal to said switch means for a selectedperiod of time. .Iadd.
 4. A signalling device according to claim 1,including a stretch means to cause said first rectified signal to havean increased pulse duration, wherein said switch means activates saidalarm during any coincident receipt of said second rectified signal andsaid first rectified signal having said increased pulse duration..Iaddend. .Iadd.
 5. A signalling device according to claim 4, includinga second stretch means to cause said second rectified signal to have asecond increased pulse duration, wherein said switch means activatessaid alarm during any coincident receipt of said first rectified signalhaving first-recited increased pulse duration and said second rectifiedsignal having said second increased pulse duration. .Iaddend. .Iadd. 6.A signalling device according to claim 1, including a stretch means tocause said second rectified signal to have an increased pulse duration,wherein said switch means activates said alarm during any coincidentreceipt of said firsts rectified signal and said second rectified signalhaving said increased pulse duration. .Iaddend. .Iadd.
 7. A signallingdevice according to claim 2, including a second delay means to delaytransmission of said second rectified signal to said switch means for asecond selected period of time. .Iaddend. .Iadd.8. A signalling deviceaccording to claim 7, wherein said second selected period of time isgreater than said first-recited selected period of time. .Iaddend..Iadd.9. A signalling device according to claim 8, including a stretchmeans to cause said first rectified signal to have an increased pulseduration, wherein said switch means activates said alarm during anycoincident receipt of said second rectified signal and said firstrectified signal having said increased pulse duration. .Iaddend. .Iadd.A signalling device according to claim 9, including a second stretchmeans to cause said second rectified signal to have a second increasedpulse duration, wherein said switch means activates said alarm duringany coincident receipt of said first rectified signal having saidfirst-recited increased pulse duration and said second rectified signalhaving said second increased pulse duration. .Iaddend. .Iadd.11. Asignalling device according to claim 8, including a stretch means tocause said second rectified signal to have an increased pulse duration,wherein said switch means activates said alarm during any coincidentreceipt of said second rectified signal and said second rectified signalhaving said increased pulse duration. .Iaddend. .Iadd.12. A signallingdevice according to claim 7, including a stretch means to cause saidfirst rectified signal to have an increased pulse duration, wherein saidswitch means activates said alarm during any coincident receipt of saidsecond rectified signal and said first rectified signal having saidincreased pulse duration. .Iaddend. .Iadd.13. A signalling deviceaccording to claim 12, including a second stretch means to cause saidsecond rectified signal to have a second increased pulse duration,wherein said switch means activates said alarm during any coincidentreceipt of said first rectified signal having said first-recitedincreased pulse duration and said second rectified signal having saidsecond increased pulse duration. .Iaddend. .Iadd.14. A signalling deviceaccording to claim 7, including a stretch means to cause said secondrectified signal to have an increased pulse duration, wherein saidswitch means activates said alarm during any coincident receipt of saidfirst rectified signal and said second rectified signal having saidincreased pulse duration. .Iaddend. .Iadd. A signalling device accordingto claim 2, including a stretch means to cause said first rectifiedsignal to have an increased pulse duration, wherein said switch meansactivates said alarm during any coincident receipt of said secondrectified signal and said first rectified signal having said increasedpulse duration. .Iaddend. .Iadd. A signalling device according to claim15, including a second stretch means to cause said second rectifiedsignal to have a second increased pulse duration, wherein said switchmeans activates said alarm during any coincident receipt of said firstrectified signal having said first-recited increased pulse duration andsaid second rectified signal having said second increased pulseduration. .Iaddend. .Iadd.17. A signalling device according to claim 2,including a stretch means to cause said second rectified signal to havean increased pulse duration, wherein said switch means activates saidalarm during any coincident receipt of said first rectified signal andsaid second rectified signal having said increased pulse duration..Iaddend. .Iadd.18. A signalling device according to claim 3, includinga stretch means to cause said first rectified signal to have anincreased pulse duration, wherein said switch means activates said alarmduring any coincident receipt of said second rectified signal and saidfirst rectified signal having said increased pulse duration. .Iaddend..Iadd.19. A signalling device according to claim 18, including a secondstretch means to cause said second rectified signal to have a secondincreased pulse duration, wherein said switch means activates said alarmduring any coincident receipt of said first rectified signal havingfirst-recited increased pulse duration and said second rectified signalhaving said second increased pulse duration. .Iaddend. .Iadd.20. Asignalling device according to claim 3, including a stretch means tocause said second rectified signal to have an increased pulse duration,wherein said switch means activates said alarm during any coincidentreceipt of said first rectified signal and said second rectified signalhaving said increased pulse duration. .Iaddend.